Biological systems nicotinamide adenine dinucleotide oxidation

Decarboxylation removal of a carboxyl group as CO2, from a ketoacid or from an amino acid. D. of ketoacids occurs several times in the course of the TH-carboxylic acid cycle (see). TTie D. of P-ketoacids often occurs spontaneously. In biological systems the oxidative D. of a-ketoacids requires coenzymes such as thiamin pyrophosphate, lipoic acid, coenzyme A, flavin adenine dinucleotide or nicotinamide adenine dinucleotide. Oxidative D. of Pyruvate (see) to ace-tyl-CoA and of a-ketoglutarate to succinyl-CoA are nodes at which many metabolic pathways cross. D. of amino acids is catalysed by Pyridox phosphate (see) enzymes. 

A number of autoxidation reactions exhibit exotic kinetic phenomena under specific experimental conditions. One of the most widely studied systems is the peroxidase-oxidase (PO) oscillator which is the only enzyme reaction showing oscillation in vitro in homogeneous stirred solution. The net reaction is the oxidation of nicotinamide adenine dinucleotide (NADH), a biologically vital coenzyme, by dioxygen in a horseradish peroxidase enzyme (HRP) catalyzed process ... 

Two derivatives of nicotinamide (pyridine-3-carboxylic amide), one of the B2 vitamins, nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ), serve as redox coenzymes. Of the three heterocyclic ring systems found in these coenzymes, i.e. those of purine, ribose and pyridine, it is the pyridine portion that is reactive in redox reactions. Biologically, two oxidation states are important the oxidized form, NAD(P)+, and the 1,4-dihydro isomer of the two-electron reduced form, NAD(P)H (Scheme 1). Nicotinamide coenzymes interconvert between these two oxidation states in... 

NADP Nicotinamide adenine dinucleotide phosphate NADP and the related NAD (the reduced forms are NADPH2 and NADH2) are coenzymes that are involved in diverse oxidation-reduction reactions in biologic systems among the processes that require NADPH2 is the metabolism of many toxicants by microsomal enzymes in the mammalian liver. 

NAD A Coenzyme Nicotinamide adenine dinucleotide (NAD) is one of the principal oxidation-reduction reagents in biological systems. This nucleotide has the structure of two D-ribose rings (a dmucleotide) linked by their 5 phosphates. The aglycone of one ribose is nicotinamide, and the aglycone of the other is adenine. A dietary deficiency of nicotinic acid (niacin) leads to the disease called pellagra, caused by the inability to synthesize enough nicotinamide adenine dinucleotide. 

In biological systems, the most frequent mechanism of oxidation is the removal of hydrogen, and conversely, the addition of hydrogen is the common method of reduction. Nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP) are two coenzymes that assist in oxidation and reduction. These cofactors can shuttle between biochemical reactions so that one drives another, or their oxidation can be coupled to the formation of ATP. However, stepwise release or consumption of energy requires driving forces and losses at each step such that overall efficiency suffers. 

Enzyme Cofactors. In many enzymatic reactions, and in particular biological reactions, a second substrate (i.e., species) must be introduced to activate the enzyme. This substrate, which is referred to as a cofactor or coenzyme even though it is not an enzyme as such, attaches to the enzyme and is most (often either reduced or oxidized during the course of the reaction. The enzyme-cofac-tor complex is referred to as a holoenzyme. The inactive form of the enzyime-cofactor complex for a specific reaction and reaction direction is called an apoenzyme. An example of the type of system in which a cofactor is used is the formation of ethanol from acetaldehyde in the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinucleotide (NAD) ... 

There are many examples of second-order reactions in biological systems. The enzyme-catalyzed reaction betweeruthe oxidized form of nicotinamide adenine dinucleotide, NAD, with L-lactic acid can be written as follows... 

In biological systems, alcohols are metabolized by oxidation to carbonyl compounds. For example, ethanol is converted into acetaldehyde by the cationic oxidizing agent nicotinamide adenine dinucleotide (abbreviated as NAD see Real Life 25-2). The process is catalyzed by the enzyme alcohol dehydrogenase. (This enzyme also catalyzes the reverse process, reduction of aldehydes and ketones to alcohols see Problems 58 and 59 at the end of this chapter.) When the two enantiomers of 1-deuterioethanol are subjected... 

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